Barrier coatings (layers) which prevent, reduce, or inhibit the permeation of a selected substrate with a gas, vapor, chemical and/or aroma have been widely described, and such coatings are used in a variety of industries, e.g., the packaging industries, automobile industries, paint industries, tire industries etc. Typical barrier materials used in coatings include polyesters, PVDC, polyurethanes, acrylic polymers, etc.
It is well known that the barrier properties of a polymer can be improved by the addition of impermeable plate like structures. When the plates are oriented perpendicular to the diffusion (permeation) direction, the diffusing molecules must go around the plates. This leads to significant reductions in the permeability of the polymer. See, for example, E. L. Cussler et al, J. Membrane Sci. 38:161-174 (1988); W. J. Ward et al, J. Membrane Sci., 55:173-180 (1991); Chang, J. et al, Journal of Applied Polymer Science, Vol. 84, 2294 (2002); Yano, K. et al, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997); Lan, T. et al, Chem. Mater. 6, 573 (1994); Messersmith, P. B. and Giannelis, E. P, Journal of Polymer Science A: Polymer Chemistry 33,1047 (1995); U.S. Pat. Nos. 4,528,235; 4,536,425; 4,911,218; 4,960,639; 4,983,432; 5,091,467; and 5,049,609; and International Patent Application No. WO93/04118, published Mar. 4, 1993, among others.
Control of permeation using relatively low aspect ratio platelets, at low concentrations, and thermoplastically processed at high shear rates has been previously disclosed. See, for example, E. L. Cussler et al, J. Membrane Sci. 38:161-174 (1988); L. E. Nielsen, Journal of Macromolecular Science, Chemistry A1,929,(1967); R. K. Bharadwaj, “Modeling the Barrier Properties of Polymer-Layered Silicate Nanocomposites”, Macromolecules 34, 9189 (2001); G. H. Fredrickson and J. Bicerano, “Barrier properties of oriented disk composites”, Journal of Chemical Physics 110, 2181 (1999). These conditions lead to relatively small improvements in the barrier properties of the polymer. This is because the reduction in permeability varies rapidly with the aspect ratio and the concentration of plates when the plates are well aligned. If the plates are not well aligned, the reductions in permeability are further reduced. The targeted application of these earlier efforts was not coatings, but a bulk polymer with improved barrier and/or mechanical properties.
Exfoliated silicates have been used to produce nanocomposite coatings by several methods. The most widely used has been by combining a dissolved polymer with exfoliated filler. Water-soluble polymers such as polyvinyl alcohol (PVOH) have been combined with water exfoliated filler such as vermiculite. See, for example, Japan patent 11-246729, Sep. 14, 1999, “Gas-Barrier Poly(vinyl alcohol)/poly (acrylic acid) Compositions and their Laminates and Shaped Articles”. Sumitomo Chemical Co., Ltd. Polycarbonate dissolved in toluene has been combined with organically functionalized filler and reportedly forms good barrier coatings. See, for example, W. J. Ward et al, “Gas Barrier Improvement Using Vermiculite and Mica in Polymer Films”, Journal of Membrane Science, 55:173-180 (1991). Other polymers have also been made into barrier coatings by dissolving them in a solvent, and using an organically functionalized filler in an effort to improve the barrier properties. See, for example, Yano, K., et al, “Synthesis and properties of polyimide-filler hybrid composites”, Journal of Polymer Science A: Polymer Chemistry, 35, 2289 (1997).
Processes for utilizing emulsion polymerization procedures for preparing aqueous polymer/clay nanocomposite dispersions are disclosed in U.S. Pat. No. 6,838,507; US Patent Applications 2005/0059769 and 2002/0086908 (all to Rohm and Haas). The disclosed processes include both in-situ polymerizations in the presence of at least partially exfoliated unmodified clays as well as admixtures of polymer dispersions with at least partially exfoliated unmodified clay dispersions. The disclosed nanocomposite dispersions are useful for preparing a variety of materials, such as coatings, adhesives, caulks, sealants, plastics additives, and thermoplastic resins. Processes for preparing polymer clay nanocomposite powders and use of these powders as plastic resins and plastics additives are also disclosed.
While nanocomposites exhibit relatively low steady state permeation rates, it has been found that significant improvement in breakthrough times can be achieved with the same nanocomposite coatweight by interposing another layer between nanocomposite layers.